The present invention relates generally to golf club heads and, more particularly, to golf club heads having an improved face construction.
Modem golf clubs have typically been classified as woods, irons or putters. Additionally, a newer class of golf clubs termed “utility” clubs or “iron woods” seek to replace low lofted long irons or higher numbered fairway woods. The term “wood” is a historical term that is still commonly used, even for golf clubs that are constructed of steel, titanium, fiberglass and other more exotic materials, to name a few. The woods are now often referred to as “metal woods.” The term “iron” is also an historical term that is still commonly used, even though those clubs are not typically constructed of iron, but are rather constructed of many of the same materials used to construct “woods”.
One particular improvement that relates especially to metal woods is the use of lighter and stronger metals, such as titanium. A significant number of the premium metal woods, especially drivers, are now constructed primarily using titanium. The use of titanium and other lightweight, strong metals has made it possible to create metal woods of ever increasing sizes. The size of metal woods, especially drivers, is often referred to in terms of volume. For instance, current drivers may have a volume of 300 cubic centimeters (cc) or more. Oversized metal woods generally provide a larger sweet spot and a higher inertia, which provides greater forgiveness than a golf club having a conventional head size.
One advantage derived from the use of lighter and stronger metals is the ability to make thinner walls, including the striking face and all other walls of the metal wood club. This allows designers more leeway in the positioning of weights. For instance, to promote forgiveness, designers may move the weight to the periphery of the metal wood head and backwards from the face. As mentioned above, such weighting generally results in a higher inertia, which results in less twisting due to off-center hits.
There are limitations on how large a golf club head can be manufactured, which is a function of several parameters, including the material, the weight of the club head, the strength of the club head, and the materials used. Additionally, to avoid increasing weight, as the head becomes larger, the thickness of the walls must be made thinner, including that of the striking face. As a result, as the striking face becomes thinner, it has a tendency to deflect more and more at impact, and thereby has the potential to impart more energy to the ball. This phenomenon is generally referred to as the “trampoline effect.” A properly constructed club having a thin face can therefore impart a higher initial velocity to a golf ball than can a club having a rigid thick face. Because initial velocity is an important factor in determining how far a golf ball travels, this is very important to golfers.
It is appreciated by those skilled in the art that the initial velocity imparted to a golf ball by a thin-faced metal wood varies depending on the location of the point of impact of a golf ball on the striking face. Generally, balls struck in the sweet spot will have a higher rebound velocity. Many factors contribute to the location of the sweet spot, including the location of the center of gravity (CG) and the shape and thickness of the striking face.
Prior golf club heads have provided an increased initial or launch velocity of a golf ball, by incorporating a lightweight, flexible face. Manufacturers of metal wood golf club heads have more recently attempted to manipulate the performance of their club heads by designing what is generically termed a variable face thickness profile for the striking face, in particular with the use of lightweight materials such as titanium alloys.
Another approach to reduce stress at impact is to use one or more ribs extending substantially from the crown to the sole vertically across the face, and in some instances also extending from the toe to the heel horizontally across the face. Because the largest stresses are located at the impact point, usually at or substantially near the sweet spot, the center of the face is also thickened and is at least as thick as the ribbed portions.
There have been other configurations and ribs formed on the back of a club face, including one or more thin rings, a power bar, and a cone formation. Multiple thin rings have been attached by various means so as to add mass directly behind the sweet spot, and alternatively a spiral formation has been used, wherein the multiple rings or spiral mass extend from the sweet spot substantially toward the periphery of the face plate. A single thin ring at the sweet spot has been used on an iron club head in conjunction with an added toe mass in order to reposition a point of least rigidity to the center of the face. In this configuration the rigidity of the face is always higher radially outward from the centered ring.
Other club heads have attempted to utilize power bars or cones behind the sweet spot in order to increase the force imparted to a golf ball. These power bars and cones involve significant additional mass extending toward a rear of the club head, thus affecting the club head's center of gravity. However, such club heads do not provide a coefficient of restitution (COR) that is at least the minimum value of approximately 0.8 that is sought by today's golfers.
The COR for a golf club may be informally defined as a function of the ratio of the relative velocities of a golf ball, just prior to and immediately after impact with the golf club head. The COR baseline value of e=0.822 has been established in the United States, and the formal equation also accounts for the relative masses of a specific club head as well as a golf ball, as follows:Vout/Vin=(eM−m)/(M+m)                (where M is the mass of the club head and m is an average mass of the golf ball population. Vout is the ball rebound velocity and Vin is the incoming velocity of the ball that is shot at the face of the golf club head using an air cannon, for example.)        
In each of the foregoing examples, however, there is ultimately a failure to provide significant forgiveness to off-center hits. Each golf club has attempted to increase COR while addressing to various degrees the difficulties in doing so. For these clubs, the point of impact must still be at the sweet spot in order for these clubs to deliver their highest COR, and even the slightest deviation of the impact from the sweet spot will result in a significant loss in ball velocity.